This invention pertains to a method for manufacturing materials suitable for use at high temperatures, particularly at temperatures and under conditions encountered in gas turbine engines, and to articles made from such materials.
The efficiency of gas turbine engines depends significantly on the operating temperature in the various sections of the engine, with increased operating temperatures resulting in increased efficiencies. The search for increased efficiencies has led to and continues to result in the development of materials which can withstand increasingly high temperatures while retaining their basic material properties, and novel manufacturing processes which permit exploitation of the capabilities of these materials. Limitations and capabilities of such alloys are generally traceable to their metallurgical structures and manufacturing processes employed in producing them.
One important aspect of metallurgical structure is active strengthening mechanisms; in nickel-base alloys, those may include solid solution strengthening, precipitation hardening and dispersion strengthening. There are commercially available alloys which exploit each of these strengthening mechanisms. Nichrome and Hastelloy X rely on solid solution strengthening. Rene'95 and IN-100 are examples of alloys which are strengthened primarily by a precipitate of the gamma-prime phase. TD Nickel is a dispersion strengthened alloy, in which thoria particles are formed by the in-situ oxidation of metallic thorium. MA 754 is a dispersion strengthened alloy which is comprised of a dispersion of yttria particles in a matrix similar to Nichrome, produced by a mechanical alloying process. There are significant limitations associated with each type of strengthening mechanism, including the extent to which strengthening can be achieved, the temperatures over which strengthening is effective and limitations in processing inherent in the alloy.
One approach to improving the capabilities of nickel-base superalloys has been reinforcing the alloys with fibers which are strong and stable at elevated temperatures. Such materials are termed metal matrix composites (MMC). In these materials, a metal matrix is reinforced with fibers of a high strength metal or nonmetallic material such as an oxide or carbide. MMC technology not only capitalizes on the high strength and stiffness of these materials, but provides the additional capability of orienting the reinforcing fibers in such patterns as may be most appropriate to carrying the loads imposed on the article in service. One of the limitations to using MMC materials is that the maximum operating temperatures are generally determined by the capabilities of the matrix material.
A special class of MMC superalloys is the directionally solidified eutectic alloys, in which reinforcing fibers, grown during directional solidification of a eutectic alloy, are embedded in a matrix comprised of a base material, such as nickel, and various alloying elements. As described in U.S. Pat. No. 4,305,761--Bruch et al., aligned metallic carbide reinforcing fibers are embedded in a nickel-base gamma/gamma-prime matrix in which the gamma-prime strengthening phase is dispersed in the gamma phase. The carbide reinforcing fibers, of the MC or monocarbide type, are those of which the metal preferably is principally Ta, but can include, in addition, such metals as Mo, W, Re and Nb. A significant limitation to the use of such materials is that the reinforcing fibers must be grown from the liquid by eutectic reaction, by directional solidification at very slow rates, typically on the order of 0.25 inch per hour, which further implies that the size of any part made by this process must be relatively small and of such configuration that the substantially parallel alignment of the fibers is appropriate.
The capabilities of superalloy articles are significantly affected by the processes employed in their manufacture. For examples, superalloys made as forgings or wrought mill products must be soft and ductile enough that they can be deformed without breaking. As a result there is a practical limitation to the strength levels usable in such materials. On the other hand, cast superalloys can be formulated to have greater strength at higher temperatures, but they are vulnerable to inhomogeneous distribution of alloying elements, shrinkage and other casting defects and, frequently, low ductility.
Novel manufacturing processes including powder metallurgy and incremental solidification processes such as rapidly solidified plasma deposition (RSPD) have been developed to circumvent some of the limitations associated with the more traditional processes. As applied to superalloys, powder metallurgy technology includes solidification of individual powder particles from droplets of liquid metal, followed by powder conditioning, hot consolidation, and sometimes forging. Powder metallurgy processing can yield materials having greater homogeneity than castings and higher strengthener content than conventionally processed wrought materials. This technique is widely used in the aircraft gas turbine engine industry for producing articles from alloys that are extremely difficult to process by other methods.
The RSPD process is one of several processes included within the term incremental solidification, in which droplets of liquid metal are brought into contact with a solid substrate maintained at a temperature low enough to cause the droplets to freeze. The droplets, typically flattened by coming in contact with the substrate, adhere to the substrate and freeze very rapidly, thereby depositing droplet material on the substrate, generally under nonequilibrium conditions. Where the substrate and the droplets have substantially the same composition, the net effect is to gradually build up a solid article of that composition. Where the substrate and the droplets have significantly different compositions, there will typically be a distinct boundary between the substrate and the deposited material; however, once a complete layer of droplet material has been made, subsequent deposition thereof proceeds in the same fashion as where the substrate and droplets are of the same composition. This sequence is particularly useful for manufacturing an article of the droplet material, where the configuration of the article is derived from that of the forming mandrel on which the droplet material is deposited. It is also useful for making an ingot from the droplet material. Each of these variations is within the scope of the term incremental solidification.
In RSPD processing, powder particles are fed into a plasma flame, where they are melted to form droplets of liquid. The plasma flame directs the stream of droplets against a substrate, where the droplets adhere and solidify. The production of superalloy articles by RSPD is described in U.S. Pat. No. 4,418,124--Jackson et al., and the prior art referenced therein; said patent is incorporated herein by reference. Another incremental solidification process is often called spray forming. The principle of spray forming differs from RSPD processing only to the extent that droplets of liquid are formed directly from the melt, rather than by remelting powder particles. Thus the difference in properties of an article made by RSPD and one made by spray forming may be negligible.
Within the context of the present invention a distinction is made between an equilibrium structure, which is reasonably stable at intended operating conditions, such as those encountered in a gas turbine engine, but which can be achieved through commercially practical heat treatment operations, and a nonequilibrium structure, which can be achieved only through special precautions to avoid what would otherwise be considered an equilibrium structure, such as, for example, a metastable structure obtained by quenching from a high temperature. A distinction is also made between a dispersoid, which is an essentially insoluble atomic, molecular or intermetallic species distributed essentially uniformly throughout a metallic matrix in the form of very small particles, and a precipitate, which is an atomic, molecular or intermetallic species distributed essentially uniformly throughout a metallic matrix in the form of very small particles, but which can be readily dissolved and reprecipitated by thermal treatment. Once a dispersoid has been developed, whether by in-situ oxidation, mechanical alloying or by the methods of the present invention, it can be altered only slightly by any solid state thermal treatment. The term dispersion is used to describe the distribution of dispersoid particles in the material.
The present invention provides several combinations of alloy composition and manufacturing method which yield materials having greater strength at higher temperature than materials heretofore employed. The present invention utilizes less conventional manufacturing processes to achieve metallurgical structures and resulting materials properties not achievable through conventional processing. Such less conventional manufacturing processes exploit the principle of first preventing the formation of, then subsequently inducing the growth of, one or more discrete phases which strengthen superalloys. Alloys of the present invention have been formulated to provide a material having good resistance to oxidation in a high velocity stream of hot air, and to raise the temperature at which superalloy and MMC articles can be used.